1. Field of the Invention
This invention relates to thermistor materials and thermistor elements. More particularly, it relates to thermistor materials suitable to form a thermistor element having a negative temperature coefficient of resistance for use at elevated temperatures.
2. Discussion of the Prior Art
Thermistors are temperature sensors which make use of the temperature dependency of electric resistance of a temperature-sensitive resistor and are widely used in measurement and control of temperature. For high temperature applications, thermistors are used as sensors for detecting the temperature of automobile exhaust gases or the temperature of electric ovens.
Materials that form temperature-senstive resistors of high-temperature thermistor elements, that is, thermistor materials are generally sintered bodies of composite oxides including fluorspar (zirconia series such as ZrO.sub.2 -CaO-Y.sub.2 O.sub.3 -Nd.sub.2 O.sub.3 -ThO), spinel (such as MgO-NiO-Al.sub.2 O.sub.3 -Cr.sub.2 O.sub.3 -Fe.sub.2 O.sub.3, CoO-MnO-NiO-Al.sub.2 O.sub.3 -Cr.sub.2 O.sub.3 -CaSiO.sub.3, NiO.sub.2 -CoO-Al.sub.2 O.sub.3, and MgO-Al.sub.2 O.sub.3 -Cr.sub.2 O.sub.3 -LaCrO.sub.3), corundum (such as Al.sub.2 O.sub.3 -Cr.sub.2 O.sub.3 -MnO.sub.2 -CaO-SiO.sub.2), perovskite and rutile structure composite oxides.
The thermistor materials based on these sintered composite oxides experience substantial changes with a lapse of time and are thus unstable for the reason that they have a crystal transformation point of lower than 1,000.degree. C. and barriers are formed between grains. Particularly, zirconia type sintered oxides experience greater changes with a lapse of time because they are oxygen ion conductors which invite redox reaction. These thermistor materials are inconsistent in resistance and performance because they are composites consisting of multiple oxides. Since these thermistor materials have a high thermistor constant B and hence, a too high temperature coefficient of resistance, thermistors formed thereof cannot cover a wide temperature range from room temperature to high temperatures. These thermistors cannot be used at temperatures of higher than 500.degree. C.
Another type of thermistor element is known in the art which uses thermistor materials based on silicon carbide and boron carbide. For example, Japanese Patent Publication No. 42-19061 discloses a thermistor element comprising monocrystalline silicon carbide having a minor amount of an element of Group 3B or 5B in the Periodic Table added as a p- or n-type impurity. This element suffers from low productivity and high manufacturing cost because monocrystalline silicon carbide must be formed. Although the element shows a very stable electric resistance at elevated temperatures, it undergoes surface oxidation when used in air at elevated temperatures, particularly at 400.degree. C. or higher. A protective film is necessary to prevent surface oxidation. The most preferred method for forming a protective film is encapsulation of a chip with glass because of ease of operation. However, the thermistor element based on monocrystalline silicon carbide tends to undergo foaming during glass encapsulation due to reaction of silicon carbide with glass. It is thus very difficult to encapsulate the element with glass.
U.S. Pat. No. 4,086,559 discloses a thermistor element comprising a pyrolyzed polycrystalline isometric silicon carbide having at most 0.7% by volume of a p-type impurity added thereto.
U.S. Pat. Nos. 4,359,372 and 4,424,507 disclose sputtered thin-film thermistor elements comprising silicon carbide or boron carbide containing a minor amount of an impurity. These thin-film thermistors suffer from low productivity and high manufacturing cost as the monocrystalline thermistors do. Glass encapsulation is substantially impossible. In the sputtered thin film of the latter patent, glass is vapor deposited to form a protective film in order to suppress foaming at the sacrifice of productivity.
Composite sintered bodies based on oxide materials and non-oxide materials are also known in the art. These are higher in productivity than the monocrystalline and thin-film thermistor materials. Among the composite sintered bodies are included the following silicon carbide-based materials.
(a) A sintered polycrystalline silicon carbide body comprising a major proportion of silicon carbide and up to 20% by weight, calculated as element, of Be, BeO, B, B.sub.2 O.sub.3, BN or B.sub.4 C (see U.S. Pat. No. 4,467,309)
(b) A polycrystalline sintered body comprising silicon carbide and 0.5 to 10% by weight of at least one member selected from aluminum and aluminum compounds such as aluminum oxide (see Japanese Patent Application Kokai No. 60-49607).
(c) A sintered silicon carbide body comprising silicon carbide having a minor amount of boron thermally diffused therein (see Japanese Patent Publication No. 60-52562).
These silicon carbide-based materials, however, are difficult to encapsulate with glass because the increased content of SiC incurs foaming. The silicon carbide-based materials are difficult to machine and thus difficult to cut into thermistor chips.
These thermistor materials also have the drawback that they have a thermistor constant B as high as 10,000K or more and hence, a too high temperature coefficient of resistance, and thus fail to cover a wide temperature range.
A study on the resistivity (.rho.) of thermistor material in relation to the ratio of components thereof reveals that the resistivity largely changes with a small change of component ratio. It is thus difficult to control the resistance of thermistor material.
Japanese Patent Application Kokai No. 55-140201 discloses a thick-film thermistor comprising a major proportion of SiC, 2 to 15% by weight of RuO.sub.2, and 20 to 50% by weight of glass. It is very difficult to control severe foaming which takes place due to reaction between powder silicon carbide and glass during printing and sintering.
Japanese Patent Application Kokai No. 60-37101 discloses a sintered material comprising silicon carbide and silicon nitride combined with a semiconductor oxide such as zirconium oxide, nickel oxide, zinc oxide, cobalt oxide, chromium oxide and titanium oxide. Also disclosed is a sintered material comprising aluminum oxide and zirconium combined with a nitride, boride, carbide or silicide of a transition element of Group 3A, 4A, 5A and 6A in the Periodic Table.
The sintered materials comprising silicon carbide and silicon nitride combined with a semiconductor oxide have several problems. (i) Since the semiconductor oxide is readily reduced during sintering, control of electric resistance is difficult. The materials tend to be affected by the ambient atmosphere because of the presence of oxygen defects. (ii) A choice of sintering conditions for composite material is difficult because the semiconductor oxides have a low sintering temperature as compared with silicon carbide and silicon nitride. (iii) Since the electric resistance is considerably lowered as a result of reduction of semiconductor oxide as described in (i), it is difficult to obtain a resistivity of several tens .OMEGA.-cm at 500.degree. C. In order to obtain a thermistor element having a resistance of 10.sup.3 to 10.sup.6 ohm as commonly used in thermistor circuits, the distance between electrodes must be increased at the sacrifice of compactness and quick response.
The sintered materials comprising aluminum oxide and zirconium oxide combined with a nitride, boride, carbide or silicide of a transition element of Group 3A, 4A, 5A or 6A are difficult to control their electric resistance. Since the nitrides, borides, carbides and silicides of transition elements of Group 3A, 4A, 5A and 6A are approximate electrical conductors, composite materials thereof with aluminum oxide and zirconium oxide drastically change their electric resistance with a slight change of composition.
Further, Japanese Patent Application Kokai No. 60-37101 discloses several thermistor material compositions. One typical example is 36%SiC-7%B.sub.4 C-55%CoO-2%Li.sub.2 O (expressed in % by weight) in which Li tends to diffuse upon application of voltage and Co is unstable at about 500.degree. C. Since this composition has a resistivity of up to 60.OMEGA.-cm at 500.degree. C., the electrode-to-electrode distance cannot be reduced, which is undesired for compactness. Other examples are 3.7%SiC-20%Al.sub.2 O.sub.3 -35%TiO.sub.2 -8%Ta.sub.2 O.sub.3 and one prepared by adding 9 parts by weight of TiO.sub.2 to 11 parts by weight of 65%SiC-35%Al.sub.2 O.sub.3. Titanium oxide which is present in a volume ratio of TiO.sub.2 to SiC of more than 1/2 is reduced into a conductor by SiC and the sintering atmosphere. The material is thus difficult to control its resistance and its resistance at 500.degree. C. is unstable. Other composite sintered bodies disclosed therein are prepared by combining at least one member of SiC, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, and ZrO.sub.2 with at least one member of NiO, ZnO, CoO, Cr.sub.2 O.sub.3, and TiO.sub.2. These materials have a problem that semiconductor metal oxides are susceptible to reduction by carbide and the ambient atmosphere, have a low electric resistance, or tend to change their valence at a temperature of higher than about 500.degree. C.